Dental furnace



2 Sheets-Sheet 2 Fig. 4.

y INVENTOR Ora Aolby. BY

ATTORNEY Jfune 16, 1931. o. A. coLBY DENTAL FURNACE AF'iled Feb.14.*1928 Patented Junev 16, 1931 UNITE- srAiss;

PATENT one A. coLBY, or M Aivsriniin; oHroAssiGNoaroWEsTINGHoUsEELECTRIC a MANU- y FACTUMNG. @M1-aime Coe-Poseer@ er' .PENNSYLVANIAprima Waseca Application filed Fbriiary 14, 1928. SerialmN'o.r254,2,08.`

invention relates to electric furnaces kand more particularly toY anelectricallyheated `furnace adapted tor use in dental-lab-` oratoriesfor the heat ytreatment or glazing.

5 of artificial teeth.

In the practi'ceyot'dentistry isV comino-n tor the dentist eitherv toconstruct special porcelainteeth or to ,purchase manufactured nporcelain teeth and` cut or grind'tliem to a` desired size and shape.:Inthe cutting and grinding operations, however, the glazed surface ot,lthe tooth is destroyed and voids are exposed, in which bacteria andfood deposits 1 may collect,.and2result in afdis'coloration ot thetooth.It becomes necessary', therefore, to apply a coatiiigtothe tooth whichmay be glazed' to cover the voids. necessary, in someinstances, to stainor color lportionsot lanartiicial tooth tomale it harmonize in colorwith the natural teeth.

All" of these procedures involver heat-treatp ment, and have usuallybeen etfected'by ein- `ploying an electric resistance Jfurnace having aheating element composedr of platinum T Wire. I n vievi7 of the inherentexpense of suchy a furnace, it 'has been attempted'to employresistance-heating elements composed of various iron alloys, andparticularly ythose containing nickelv and chromium, but the ap 301plications have invariably proved lto be iin-` successlful. I r

Various methods of construction have been.

employed in attempting to utilize a nickelchromium alloy torthispurpose.v In one con- #struction the resistance ele-ment'vvas com letelyseparated trom the' Workto be heated,

by a muifle substantially covering the side Vv Wallsy ofthe heatingchamber. This method proved objectionable in view otfthektim'e reoquiiedto bring the turnaceup to operating temperature, Which undesirably ylengthened the time of the cycles of operationg In an-V otherckonstllction the heating element-Was embedded `in the side Walls' ofthe furnace' l chamber, l but a the relatively. high Qimatingtemperature v desired (approximately 1950b F.), the element rapidlyburned out land the construction 'provedto be impralo ticable from acost standpoint.4

of In another construction, the heating ele- Further, it is l ment Wasso disposed upon the Walls of the heating chamber that the Work to beheated. was directly exposed to heat radiating'therefrom. Thisconstruction Was found to be obvjectionable because theelement spalled,re-

sponsive to repeated heating andcooling', and flakes of material casttherefrom stained andi vspotted'the light-colored porcelain, resultingthe vvorlto be heated rWhich is removable ktroni the heat-ing chamber,and which hasa Y suiiiciently low lieatstorage capa-city Where-j` K'bythe vvorlrniay be enclosed 'and protected duringkthe heating andcooling operationsy "Without a substantial increase 'in time in thecycles ,otoperatiom Y In practicing 4my inveiition,-I provide a4relativelysmall heating chamber, means for l supporting anickel-'chromium resistance ele-,

ment onthe longitudinalvvalls and floor thereof, vmeans forV minimizingthe spalling ofv 'materialfrom the Walls and resistanceelement', andanimprovedmeans forprotecting the toothduring' the heating and coolingoperations Y l In the accompanying drawings,

Figure l is a viev'v in front'A elevation l a'dental furnaceembodying'my invention,

FigfQ is a view in vertical longitudinal sec? tion taken on'thelineII-II of Fig. l, I l y Fig. 3 is a View in en d elevation of aresistor support Iconstituting apart of my invention,

Fig. 4 is a .view in longitudinal section tak-` en onthe line Ivy-IV ofFig. 3,

Fig. I5 iS. tray, i y n Fig. 6 is a view in `longitudinal section ntaken on the line YI-VI of Figa, Fia Y plan View .of e werk-Brosses 95plan View ,0f a Work-Supporting cover adapted to cooperate with saidtra-y, and

Fig. 8 is a` View in vertical section taken on the line VIII-VH1 of Fig.7.

Referring more particularly to the drawings, a furnace embodying myinvention comprises a pair of cast metallic plates 1 and 2 and ametallic shell 3. disposed therebetween, constituting` a metallicfurnace casing. The lower portion of the casing is closed by a plate 4,of electric-'insulating material, constituting a terminal block ashereinafter described. The casing is maintained in assembled relation bymeans of a plurality of clamping bolts 5 extending longitudinallytherethrough. rllhe cast-metallic plate 1 is provided with an opening toafford access to the interior of the casing. n

lVithin the casing are disposed a plurality ofblocls 7 and 8 of moldedceramic heat-insulating material defining a furnace chamber 9. Y

ThebloclrS comprises an arched portion 10, constituting the roof andsidewall portions ofthe chamber, and a flat horizontallyV eX- tendingportion 11 integral. therewith, constituting the floor of the chamber.

A plate 12 composed of a close-grained highly refractory material, suchas porcelain, is partially embedded in the inner face of the block 7 andis substantially coeXtensive in area with the lateral dimensions of thefurnace chamber. The block 7 is molded in a single piece and the plate12, which has been preformed, is embedded therein in the moldingoperation. rlfhe bloclrand plate are then vbaked as a unit and theresulting structure is a (substantially unitary block having a portionof one face thereof composed of highly refractory material. l

rlhe block 8 is enlarged adjacent the forward portion thereof andextends a short distance toward the longitudinal axis of the furnace, asshown more particularly in Fig..2, .md a facing' 13 of porcelain isdisposed peripherally of the Yedge of the opening defined thereby. Theporcelain is applied to the bloclr 8 during the moldingoperation'thereof, and the result, as in the case of the bloclr'7,ris aSubstantially integral structure.

The opening definedby the forward edge of the blocks 8 cooperates withthe opening in the cast plate 1 to permit charging and.

removing material with respect to the furnace `chamber. n I

The charging opening is normally closed during the heating operation bya bloclr 14 of heat-insulating material which is provided with a facing,or cap 15, of porcelain, molded ipon the inner face thereof in a mannersimilar to the porcelain facings 12 and 13, Vh-en in operative positiontheclosure 14 fits relatively closely in the furnace opening and is fsupported upon a flange 17 cast integral with vthe plate 1*;Manipulation ofthe closure is facilitated by means of an enlarged rib,extending vertically thereof, constituting a liandle portion. A peephole18 extends through the closure block 14, to permit the observation ofconditions in the furnace chamber, and may be normally closed, ifdesired, by a removable refractory plug (not shown) in a well-knownmanner.

The heating element 20 comprises an electrical-resistance wire ofrefractory metallic material, such as a nickel chromium alloy, wound inthe form of an elongated helix. The specific form of the element isobviously immaterial and may be varied as desired to meet therequirements of a particular installation.

The heating element 2O yis supported in operative position by means of ashell 21,V shown in detail in Figs. 3 and 4, composed of a close-grainedhighly refractory material, such as porcelain, and provided with aplurality of horizontally extending grooves on the inner surfacethereof. rllhe heating element is threaded through the grooves andoperatively supported therein.

The grooves are somewhat restricted in lateral area adjacent the innersurface of the shell and thereby serve the double purpose of maintainingthe heating element therein, and of reducing to a minimum the amount ofmaterial cast from the element into the lieatino' chamber, responsive tothe spalling thereof, consistent with the obtaining of a desired amountof direct heatradiation from the elem-ent.

rlhe outer dimensions of the shell 21 are such that the roof, ylioor andside walls of the heating chamber, defined by blocks 7 and 8, arecompletely covered and the ends of the shell cooperate with the plate 11and porcelain facings 12, to constitute a heating chamber, the walls ofwhich are composed of a close-grained thermally-insulating materialhaving highly` refractory characteristics. Y

rlhe heating element may comprise a plurality of sections of resistorwire adapted to be independently connected in an electric circuit toeffect a desired temperature control,

but l prefer, in the present embodiment to employ a single continuouselem-ent. yThe ends of the element are connected, respectively, to twoterminal po ts 22 secured to the terminal block 4.

toggle snap switch 23, of a usual type, is

power, are connected,respectively,to the other terminal of switch 23,andto the other ter- 'insides-'3 minal post 22, and extend through suitableopening in the cast plate 2. The opening isY bushed to protect the leadsagainstabrasion, in the usual manner.

With the construction described, the ref `in the present embodiment, issupported Within the heating chamber upon a removable tray 28 which isconstructedof porcelain,

or other suitable highly refractory material; The tra-y is provided witha vertical flange or bead 29, upon the upper surface thereof,

l adapted tosurround the Work, andflaterally I extending beads 8O on theunder 'surface' thereof, constituting spaced supports for the tray. @neend of the tray is elongated, and in a plane substantially higher thanthe body of the tray, to provide means for facilitating the handlingthereof incident to mcvingthe same into and out of the heating chamber.

A cover, or cap, 3l, comprising relatively thin Walls of highlyrefractory material, is

substantially co-eXtensive in horizontalsection With the outer dimensionof the bead 29,

, surface of the lug 32, and is adapted tok contain a temperatureindicating` means, such as e and is adapted to cooperate therewith tocompletely enclose 'the Work. c Y Y A projecting fla-nge or lug 32 isVformedA integral With the cover to facilitate handling thereof, andahorizontally extending flange f or rim 38 isformed on the lowerperipheral edge of the cover. l

A depression 34: is provided inthe upper a gold pellet, as hereinafterdescribed.r

It is to be. understood that the tray 28 and cover 3l are of very thinmaterial and althoughV offering substantially no hindrance to ,the iioivof heat therethrough, arey sui'iciently refractory to withstand therelatively high operatingtemperature of the furnace chamber'.

- In operation, the artificial tooth to be heat` treated or glazed issuitably supported'upon the tray 28 and the cover 3l is placed in oper-.itive position.` A fusible material, preferably a piece of gold foil,is placed in the dei pression 34 in the cup, and the tray and cup areplaced' in the heating chamber, Which been heated to a desired degree.-

has

It is desirable, inthe present ,'instance, to subject the Work to atemperature of approximately 1950f F.,and, since this temperature isapproximatelyy the melting point of geld, the melting of the gold foilin the depression 34 aEords a reliable indication ofthe attainjment ofthe proper operatingtenifperature.

'Thel conditionl of the gold foi-l may be observedthroiigh-:thepeephole-l8 in the closure block' 14.

kUpon the attainment of the desiredtemperature,.lthe closurel block isremoved and the lWork-supporting. tray and-cover are removed frointhechamber and permitted to cool freely `in the air". The cover may bemaintainedf'in operati-ve position, during a Y. portion'of-the coolingperiod to-fprevent Vcracking of the surface of the heatedgtoothresulting fronithe contact `of cold air there- With.-

cast/fromthe heating element and-chamber Walls, and,'in View of thefinely divided1character thereof, contamination of the Work therebyisprevented With-difliculty. By constructing the furnace ch-amber Walls ofa close-grained porcelain, rand employing substantially non-oxidizableheating element, the .amountfof materialr spalling therefrom may beeectively reduced."{.Irrespective of the v'qualityqof the materials,however, a certain amount of. spalling occurs 'and theparticles cast olfare circulated4 through@V the chamber by convection currentstherein...1v

:By ,myinventiom however, the Work is maintained out of contact Withthefurnace i `chamber atmosphere at allv times' and the Work is adequatelyprotected from the rela.-

tivelyvsmall amount of spalled'materia'lcin culating in the; furnacechamber atmosphere.

` 1 Although the'heat-storage'effect of the tray aiid cover rissuflieiently small tov offer sub-y stantially no4 resistance to :the.iioW 'of heat therethrough, itis sufficient to maintain a small 1'positive' pressure vvithinftheY ffc'over,v

, Which,pressurer further 'tends to Ypreventv contamination of the Workby particles fspal'ling r from th'e'heating element and furnace-chainbe-rWalls.. l r'. @i f, l, The cover 'is maintained in operative posi-y`tionsubsequent tothe heating operation, so

that the Work is protectedy against atmospheric impurities, and at thesame time, a tooV rapidcooling of the surface of thetooth is prevented."'i n Obviously, itis Within the scope of th-e present invention to varythe structural `details, of the furnace described, tomeet the re#quirements of a particular installation, or ap# plication, of theinvention. n The furnace structure Vproper is being claimed in thisapplication, and the material container'is being claimed in a divisionalapplication. l l

Various modifications may be made in my invention Without departing fromthe spirit scope thereof, and I desire, therefore, that onlysuchlimitation shall be placed thereon as are set forth bythe prior artand imposedr by the-appended claims.

' wDuringtheheating operationlparticles are 1731.9

aufv4 VI claim as my -invention:

l. In a furnace, a furnace wall including a relatively thick body ofheat-insulating ceramic material, and a relatively thin embedded plateof porcelain constituting a substantially nou-spelling heat-resistantface for said Wall and molded substantially integral therewith.

2. `In a furnace, a heating chamber and a charging openingcommunicating` therewith,

a door Vof molded ceramic material normally closing said opening, and arelatively thin facing of substantially non-spalling material on theinner surface of said door molded substantially integral therewith andof a higher degree of .refractoriness than the molded ceramic material.Y Y

3. In an electrically heated dental furnace, a plurality of wallsdefining a heating chamber having a charging opening communicatingtherewith, a closure for said opening, means constitutinga relativelythick lining of ceramic heat resistant refractory material disposed tocompletely cover the roof, floor and side walls ofk said chamber, and afacing of substantially non-spalling porcelain molded integral with theinner face of said closure and'peripherally of said opening.

4. In an electric furnace including a plurality of relatively thickwalls of ceramic heat-resisting material Vdefining a furnace chamber, ashell of substantially non-spalling porcelain material arranged to coverthe top, floor and side Walls of said chamber, a plurality of groovesformed in the inner surface of said shell for receiving portions of aresistance element, relatively thin porcelain plates embedded: in theend portions of the walls cooperating With said'shell to constitutesubstantially a non-spalling walls defining said chamber.

In testimony whereof, I have hereunto sub-V',

scribed my name this 9 day of February, 1928.

' i ORA A. COLBY.

